1. Field of the Invention
The present invention relates to an image pickup apparatus using an image pickup means subjected to nondestructive read access. More particularly, the present invention relates to an image pickup apparatus capable of appropriately processing an output from an image pickup means having a wide dynamic range.
2. Related Background Art
In recent years, extensive studies have been made on photoelectric transducers, and especially CCD and MOS solid-state image pickup devices.
According to the principle of operation in a conventional CCD image pickup device, a potential well is formed under a MOS capacitor electrode, charges are generated in the well upon light radiation, and the potential wells are sequentially moved to transfer the accumulated charges to an output amplifier in response to pulses applied to the capacitor electrode in the read mode. Therefore, the CCD image pickup device has a relatively simple structure and low noise generated by the CCD itself. In addition, an amage of a low light intensity can be easily read.
According to the principle of operation in a conventional MOS image pickup device, charges are respectively accumulated in p-n junction photodiodes constituting a light-receiving unit upon light radiation, and the charges accumulated in the photodiodes are transferred to an output amplifier by sequentially turning on MOS switching transistors connected to the photodiodes. Although the MOS image pickup device is more complex than the CCD image pickup device, the accumulation capacity can be large, and hence the dynamic range can be widened.
However, these conventional image pickup devices have the following disadvantages which obstruct future developments of high-resolution image pickup devices.
The conventional CCD image pickup device has disadvantages: (1) since a MOS amplifier as an output amplifier is formed on a single chip, 1/f noise that appears as noticeable noise on an image is generated from an interface between silicon and a silicon oxide film; (2) if the number of cells is increased to obtain a high packing density so as to increase a resolution, a maximum amount of charge accumulated in one potential well is reduced to narrow the dynamic range; and (3) the presence of even one defective cell inhibits charge transfer to decrease the product yield since the CCD device has a structure for transferring the accumulated charge.
The conventional MOS image pickup device has disadvantages: (1) a wiring capacitacce is coupled to each photodiode to generate a large signal voltage drop in the read mode; (2) the wiring capacitance is large to cause generation of random noise; and (3) fixed pattern noise caused by variations in parasitic capacitances of scanning MOS transistors avoids image pickup operation at a low light intensity. In addition, when the cell size is reduced to obtain a higher resolution, the accumulated charge is reduced but the wiring capacitance is not greatly decreased. As a result, the S/N ratio is decreased.
As can be apparent from the above description, the conventional CCD and MOS image pickup devices have high resolution problems. In order to solve these problems, semiconductor image pickup devices employing a new and improved system are proposed (Japanese Patent Disclosure Nos. 150878/1981, 157073/1981, and 165473/1981). According to this system, charge generated upon light radiation is accumulated in a control electrode (e.g., the base of a bipolar transistor, or the gate of an electrostatic induction transistor SIT or MOS transistor). The accumulated charge is amplified and read out by utilizing the amplification function of each cell. In this system, a high output can be read out, the dynamic range can be widened, noise can be reduced, and nondestructive read access can be achieved. Therefore, this system has a possibility for a higher resolution.
A phtoelectric transducer signal is read out a plurality of times in the nondestructive read mode, the amplitude of the readout signal is undesirably decreased because the charge accumulated in the base of the photoelectric transducer element is discharged by a base current in the signal current read mode. The decrease in charge can be approximated by a relationship between a static forward current transfer ratio h.sub.FE, an equivalent base capacitance C.sub.B, and an emitted load capacitance C.sub.V of the photoelectric transducer element: EQU .epsilon..varies.C.sub.V /(h.sub.FE .times.C.sub.B)
For example, if h.sub.FE =2000, C.sub.B =0.1 pF, and C.sub.V =4 pF, then one read cycle causes a 2% decrease in charge. When the nondestructive read cycle is repeated a plurality of times, the amplitude of the output signal from the image pickup device is decreased according to the number of read cycles. Assume that combinations of horizontal lines of the image pickup device are changed to read out signals from the image pickup device by utilizing the above nondestructive read mode. For example, the readout signal of the first cycle is used as a first field signal, and the readout signal of the second cycle is used as a second field signal to form a still image. When this still image is displayed on a display, differences between signal levels in units of fields cause flickering, thus resulting in poor display.
In addition, assume that a moving object is recorded as a movie image by a conventional image pickup/recording apparatus, and that the movie image is observed as a one-frame still image in the reproduction mode. In this case, since the recording periods between the two adjacent fields are lagged by 1/60 sec., the resultant still image undesirably blurs. In order to form a perfect still image, signals of two fields must be simultaneously obtained by using a means such as a shutter. The signals of two fields must be sequentially read out, and thus a large number of pixels are required. Therefore, when a movie image is to be recorded, flickering and variations in signal level must be minimized.
Each pixel of the image pickup device has an amplification function, and thus a very wide dynamic range can be advantageously obtained.
The above image pickup device can pick up an image in the range of high to low intensities without requiring an illumination lamp.
Although the image pickup means has a wide dynamic range as an advantageous feature, a signal processing system for processing an output from the image pickup means has a narrow dynamic range and cannot appropriately process the signal from the image pickup means, thus resulting in poor reproducibility. In a conventional signal processing system, an output from the image pickup means must be attenuated by an attenuator. In this case, the S/N ratio of the low level signal is undesirably reduced.